The present invention relates to a method and apparatus for stripping ammonia from an alkaline regenerant solution used for the regeneration of a zeolite ion exchange bed.
Ammonium ions interfere with many of the important uses of water. They are toxic to fish, corrosive to metals and concrete, and a matter of concern when consumed by man. These ions enter water supply systems from a variety of sources, one major input being the discharge of municipal and industrial wastewaters with high ammonium ion concentrations. Most sanitary discharges contain 15 to 30 parts per million (ppm) ammonia as nitrogen (typically 20 ppm). A generally accepted goal of the industry is to reduce this ion level to the lowest value commensurate with economy and good engineering practice (0-3 ppm). This level can be achieved by the regeneration process embodied in the present invention.
Traditionally, emphasis in wastewater treatment has been placed on the removal of biologically degradable organic material, suspended solids, and floating substances. The objectives of such treatment are to produce a clear effluent, which, when mixed with the receiving water, will produce minimal oxygen depletion and no gross signs of pollution or objectionable odors. The physical and biological treatment processes developed to achieve these objectives do not reduce ammonium ion concentrations to desirable levels. Therefore, some form of ammonia removal is necessary prior to the discharge of the wastewater. Considerable attention has been directed to the effective and economic removal of ammonia nitrogen from wastewater streams.
Nitrogen can be removed through microbial action on conventional biological waste treatment plants. Removal using the standard activated sludge process requires a sufficient mean cell residence time to allow nitrification bacteria to become established in the system. The required aeration period length negates the economic advantages of high rate biological systems. Even in a compartmentalized system, treatment periods are long and problems develop in maintaining systems with different biological functions. In addition, algal harvesting or stripping requires more land than other plant processes. Biological nitrification-denitrification has proven erratic and inadequate to meet water quality criteria.
The uncertainties and costs of biological removal have stimulated the investigation of physical/chemical removal of ammonia nitrogen by an ion exchange process. Ion exchange ammonia removal is more amenable to control than biological processes and more adaptable to the fluctuating flows and concentrations of municipal wastewater systems. As a unit process, ion exchange is easily controlled to achieve almost any desired product quality. In the ion exchange process, the ion exchange material exchanges the ammonium ion (NH.sub.4 .sup.+) in the waste stream for ions originally in the ion exchange bed.
The ion exchange process to which the present invention is directed, uses a zeolite bed, such as clinoptilolite (a natural zeolite), through which a clarified sewage effluent flows. As the influent passes through the bed, NH.sub.4 +ions lodge on active sites on the zeolite. The exchange medium preferentially absorbs ammonium ions in the presence of sodium, calcium and magnesium ions. A complete discussion of the use of a clinoptilolite ion exchange bed for the removal of ammonia may be found in a report approved for publication by the Environmental Protection Agency entitled: Optimization of Ammonia Removal by Ion Exchange Using Clinoptilolite by the Sanitary Engineering Research Laboratory, College of Engineering and School of Public Health, University of California, Berkeley (September 1971).
The removal of ammonium ions through ion exchange has proven to be the simplest part of the job. The technically challenging aspects come in regenerating the ion exchange bed and disposing of the regenerant. When the clinoptilolite bed becomes exhausted with ammonia, it is necessary to regenerate the bed by passing an appropriate regenerant solution therethrough. Heretofore proposed regenerant solutions have been composed of various concentrations of NaCl or CaCl.sub.2, and NaOH or Ca(OH).sub.2 adjusted to a pH of about 10.5. The regenerant passes through the exhausted bed to replace the ammonium ions with sodium or calcium ions. This regeneration also chemically changes the NH.sub.4 +to NH.sub.3. Heretofore, it has been the practice to either discard the regenerant solution after one use or reuse it only after stripping the ammonia from it. When the regenerant solution is used only once, it adds significantly to the cost of the process. The present invention is directed to a unique method and apparatus for stripping ammonia from the regenerant solution.